The present invention relates to a particular xe2x80x9cno tuningxe2x80x9d filtering structure in rectangular wave guide with bends, for the connection of an antenna to a transmitter and a receiver. In the specific technique, the filtering structure is identified by the term xe2x80x9cantenna duplexerxe2x80x9d and in particular it is in a device enabling the simultaneous use of a same antenna both by the transmitting and by the receiving equipment. In general, the operating frequency of the transmitting equipment differs from the operating frequency of the receiving equipment.
It is known that antenna duplexers for radio links, generally include a rectangular wave guide having three opening sides or ports. A band pass filter is associated with each one of the two side ports. The band pass filters are tuned to the transmitter and receiver operating frequency, respectively. A xe2x80x9cTxe2x80x9d junction is associated with the central port (or antenna port), so that a side port and the antenna port are electrically matched, for example, on a first frequency band coinciding with the operating band of the transmitter. The other side port and the antenna port are matched on a second frequency band coinciding with the operating band of the receiver.
Therefore, the subject duplexer has the function to convey the received signals from the antenna towards the receiver and also to send the transmitted signals to the antenna.
In particular, duplexers with bends are configured in such a way to have two mentioned side ports above on a side of the guide, thus enabling a considerable reduction of the overall dimensions of the whole transmission system.
Band-pass filters, in a rectangular wave guide, are usually made by resonant cavities inside the guide, which are coupled among them through physical discontinuities obtained inside the guide itself.
A type of discontinuities used to implement these band pass filters, is a series of thin traverse metal inserts (the relative filters are therefore identified by the name xe2x80x9cmetal insertxe2x80x9d filters) that cross some portions of the cavity of the wave guide. The filtering characteristics are determined by the length of the metal inserts and by the distance between the metal inserts. Therefore, it is clear that to obtain the best filtering characteristics, the accuracy in the realization and the positioning of the metal inserts inside the wave guide are of particular importance.
This accuracy of realization and assembling is obtained by manufacturing the metal inserts in one piece only, with a rectangular frame inserted in the wave guide along the longitudinal midplane parallel to the wall where the opening sides are located. The wave guide is subdivided into two half shells. The rectangular frame including a plurality of inserts and openings is placed between the two half shells. The plurality of inserts and openings behave as resonant cavities placed in such a way so as to perform the desired filtering function.
It must be considered that the above mentioned rectangular frame can be manufactured through highly accurate mechanical techniques, such as for example electroerosion and the like. These manufacturing technologies achieve an accuracy in the range of microns, which is an accuracy higher than the allowable margin of error. The allowable margin of error is in the range of a hundredth of millimeter.
Metal insert filters and multiplexers are disclosed in the following references:
IEEE Transaction Microwave Theory and Techniques vol. 37 No 2 February 1989 xe2x80x9cRigorous Field Theory Design of Millimeter Wave E-plane Integrated Circuit Multiplexers.xe2x80x9d, by Joachin Dittlof, Fritz Arndt.
IEEE Transaction Microwave Theory and Techniques vol. 36 No 12 December 1988, xe2x80x9cComputer Aied Design of Slit Coupled H-Plane T-junction diplexers with EPlane Metal Insert Filters.xe2x80x9d by Joachin Dittlof, Fritz Arndt.
IEEE Transaction Microwave Theory and Techniques vol. MTT 30 No 2 February 1982, xe2x80x9cTheory and Design of Low Insertion Loss Fin Filters.xe2x80x9d by J. Bornemann, Fritz Arndt.
However, none of the above-noted references disclose a duplexer with bends.
In known duplexers with bends, the ending parts of the internal cavities of the wave guide close on a short circuit, made of the end walls of the wave guide itself.
It must be considered that the bend is electrically matched only if the above mentioned short circuit is placed at a well defined distance from the axis of the relative side port. This result will be more clear hereafter, by making reference to FIG. 2.
However, it must be kept in mind that the end walls of the wave guide are made through traditional mechanical working, such as for example milling operations and even using high accuracy numeric control equipment it is not possible to obtain the accuracy required for the subject applications which, as said above, must be in the range of a hundreth of millimeter.
A recurring problem of known duplexers is the difficulty to space the above mentioned end wall of the guide with the required accuracy from the axis of the relative port. Another technical problem is to compensate the negative effects derived from the bending radius of the mill itself. Although a small size can be used, it always cause a working radius between the internal walls of the cavity forming the wave guide itself.
These working tolerances negatively alter the filtering characteristics of the signal inside the wave guide, particularly in relation to the portion of the cavities adjacent to an end wall that is used as resonator. In fact, in these conditions the working error of the mill highly influences both the tuning frequency of the bend (it is a problem for the electrical bend matching) and the frequency tuning of the above mentioned filter (when the last cavity of the filter actually coincides with the bend itself).
In conventional designs tuning screws are generally employed, that are allowed to penetrate at least in part the wave guide. Through adjustment of the penetration depth it is possible to compensate for the negative effects derived from the above mentioned working tolerance. However, it must be kept in mind that the presence of these screws involves the introduction of a tuning step in the production process and consequently the addition of a non negligible cost item.
It is an object of the invention to overcome the disadvantages of the prior art by implementing a duplexer whose filtering characteristics are not affected by the mechanical tolerance of the working of the internal cavities of the guide itself, so that it is possible to avoid the use of the mentioned tuning screws and realize a so-called xe2x80x9cno tuningxe2x80x9d duplexer.
The above object is solved by the combination of features of the present invention.
Therefore, considering that in known type duplexers with bends the two end walls of the wave guide form a short circuit, an embodiment of the invention comprises closing the cavity on a inductive load instead of on a short circuit. In particular, the inductive load is realized through the insertion in at least one end of the guide of an additional section of guide, xe2x80x9cunder the cut-off frequencyxe2x80x9d, in which the size of the guide does not enable the signal propagation.
According to aspects of the invention these guide sections are realized through insertion in the guide of a metal insert, essentially identical in thickness and position to that used in making the filter mentioned above.
It is evident that if the additional metal insert is made during the manufacturing process of the metal inserts forming part of the filtering structure mentioned above, and in particular if a unique frame is realized, including both the metal inserts of the filtering structures and the additional metal inserts forming the above mentioned inductive load, it is possible to take advantage of the benefits derived from the use of the electroerosion process to nullify the negative effects derived from the mechanical tolerance related to the above mentioned milling operation.
Actually in these sections of guide xe2x80x9cunder the cut-off frequencyxe2x80x9d, the signal has an exponentially attenuation in its propagation from the relative side port towards the end walls or short-circuit. Once the signal has reached (highly attenuated) the end walls, it is reflected and has an additional exponential attenuation during propagation in the opposite direction. The negative effects derived from the above mentioned working tolerance are more attenuated as the length of the two additional sections of guide xe2x80x9cunder the cut-off frequencyxe2x80x9d increases.
The new critical distance to be considered is therefore the one existing between the axis of the port and the above mentioned additional metal insert, which as said above, can be manufactured through electroerosion with a margin of error lower than the maximum accepted one.
Therefore, the advantages derived from the electroerosion technology to electrically match both the above mentioned filter and the bend (i.e., no technological difference exists between filter and bend).